Modulator for frequency-shift keying of optical signals

ABSTRACT

Described are an FSK modulator and a method for large-alphabet FSK modulation. The FSK modulator and the method are based on filtering of a multi-tone optical source such as a mode-locked laser which provides a comb distribution of tones. A frequency-selective component selects for transmission a subset of the tones. In various embodiments the frequency-selective component is a Mach-Zehnder interferometer filter or a microring resonator filter. A second frequency-selective component selects a subset of the tones from the comb distribution provided by the first frequency-selective component. Still more frequency-selective components can be used according to the number of tones supplied by the multi-tone optical source to the FSK modulator. The optical signal exiting the last frequency-selective component includes only a single tone which corresponds to the symbol to be transmitted.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/530,660, filed Sep. 10, 2009, titled “Modulator for Frequency-ShiftKeying of Optical Signals,” which is the National Stage of InternationalApplication No. PCT/US08/56012, filed Mar. 6, 2008, titled “Modulatorfor Frequency-Shift Keying of Optical Signals” which claims the benefitof the earlier filing date of U.S. Provisional Patent Application Ser.No. 60/895,756, filed Mar. 20, 2007, titled “High-Speed Modulator forFrequency-Shift Keying,” the entireties of which are incorporated byreference herein.

GOVERNMENT RIGHTS IN THE INVENTION

This invention was made with United States government support underContract No. FA8721-05-C-0002 awarded by the United States Air Force.The government may have certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to optical communications basedon frequency-shift keying (FSK). More particularly, the inventionrelates to large-alphabet FSK modulation at high symbol rates using aserial configuration of filter stages based upon Mach-Zehnderinterferometers or microring resonators.

BACKGROUND OF THE INVENTION

Power efficient optical links are useful for a number of applicationssuch as high-bandwidth free-space communications. The most efficientoptical communications links utilize large-alphabet orthogonalmodulation formats such as pulse-position modulation or frequency-shiftkeying (FSK) modulation. For FSK modulation, a transmitter transmits kbits of information by sending one of M possible frequencies during eachsymbol period where M=2^(k). The receiver determines which one of the Mfrequencies was transmitted in order to receive the k bits ofinformation.

FIG. 1 shows a generic configuration for an FSK transmitter 10. Thetransmitter 10 includes a transmitter source 14, an FSK modulator 18, anoptical power amplifier 22 and transmit optics 26. The FSK modulator 18is used to select one of M optical frequency components (i.e., “tones”)in the optical signal generated by the transmitter source 14. The poweramplifier 22 increases the optical power of the signal transmitted bythe FSK modulator 18 and the transmit optics 26 are used to conditionthe optical signal for transmission to one or more receivers. Forexample, the transmit optics 26 can include optical components toachieve a desired beam geometry for the FSK-modulated optical signal forfree-space transmission or for launching into an optical fiber.

There are several techniques that have been demonstrated fortransmission of optical FSK waveforms. As shown in FIG. 2, a tunablelaser 30 can be used as the combination of the transmitter source 14 andFSK modulator 18 of FIG. 1. The tunable laser 30 is tuned to a singletone by varying a bias current or by changing the characteristics of thelaser cavity. The illustrated configuration 10′ is typically limited tosymbol rates of a few GHz or less. An alternative configuration 10″ isshown in FIG. 3 and includes a number M of source lasers 32A to 32M(generally 32) where each source laser 32 operates on a unique tone.Each source laser 32 is intensity modulated by direct modulation of thelaser current or, as shown, using an external modulator 34A to 34M(generally 34). The external modulators 34 are activated by controlsignals so that an optical signal from only one source laser 30 istransmitted during a symbol period. The optical signals exiting theexternal modulators 34 are combined along a single optical path by acombiner 36 (e.g., a wavelength division multiplexing (WDM) combiner)although during normal operation only one of the external modulators 34permits its optical signal to be transmitted to the combiner 36.

While the transmitter configuration 10″ illustrated in FIG. 3 is usefulat high symbol rates (e.g., rates exceeding 40 GHz), the complexity ofthe transmitter is impractical for a large number M of source lasers 32.For example, if the number M of source lasers 32 in the transmitter is1,024, the number of external modulators 34 required is 1,024.

SUMMARY OF THE INVENTION

In one aspect, the invention features an FSK modulator for opticalcommunications. The FSK modulator includes a first filter stage and asecond filter stage. The first filter stage has an input optical path,an output optical path and a first microring resonator. The inputoptical path receives a multi-tone optical signal. The first microringresonator has a first optical path length and is disposed between theinput and output optical paths. An electro-optic modulator is disposedon the first microring resonator to modulate the first optical pathlength in response to a first control signal. A first filtered opticalsignal having a subset of the tones in the multi-tone optical signal isprovided in the output optical path of the first filter stage. Thesecond filter stage has an input optical path in optical communicationwith the output optical path of the first filter stage, an outputoptical path and a second microring resonator. The second microringresonator has a second optical path length and is disposed between theinput and output optical paths of the second filter stage. Anelectro-optic modulator is disposed on the second microring resonator tomodulate the second optical path length in response to a second controlsignal. A second filtered optical signal having a subset of the tones inthe first filtered optical signal is provide in the output optical pathof the second filter stage.

In another aspect, the invention features an FSK modulator for opticalcommunications. The FSK modulator includes a first filter stage and asecond filter stage. The first filter stage has an input optical path,an output optical path and a first microring resonator. The inputoptical path receives a multi-tone optical signal. The first microringresonator has a first optical path length and is disposed between theinput and output optical paths. At least one electro-optic modulatorelectrode is disposed on the first microring resonator to modulate thefirst optical path length in response to at least one binary controlsignal. A first filtered optical signal having a subset of the tones inthe multi-tone optical signal is provided in the output optical path ofthe first filter stage. The second filter stage has an input opticalpath in optical communication with the output optical path of the firstfilter stage, an output optical path and a second microring resonator.The second microring resonator has a second optical path length and isdisposed between the input and output optical paths of the second filterstage. At least one electro-optic modulator electrode is disposed on thesecond microring resonator to modulate the second optical path length inresponse to at least one binary control signal. The numbers ofelectro-optic modulator electrodes in the first and second microringresonators are different and a length of at least one of theelectro-optic modulator electrodes of one of the microrings is less thanthe lengths of the electro-optic modulator electrodes of the othermicroring. At least one tone of the multi-tone optical signal isselected for transmission through the first and second filter stagesaccording to the binary voltage signals applied to the electro-opticmodulator electrodes of the first and second microrings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood by referring to the followingdescription in conjunction with the accompanying drawings, in which likenumerals indicate like structural elements and features in the variousfigures. For clarity, not every element may be labeled in every figure.The drawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the invention.

FIG. 1 is a high level block diagram of an FSK transmitter.

FIG. 2 illustrates an FSK transmitter configuration as is known in theart.

FIG. 3 illustrates another FSK transmitter configuration as is known inthe art.

FIG. 4A illustrates an embodiment of an FSK modulator according to theinvention.

FIG. 4B illustrates the optical spectrum of a multi-tone optical signalpropagating from a transmitter source to a first filter stage in the FSKmodulator of FIG. 4A.

FIG. 4C illustrates the tones in the optical signal received at a secondfilter stage in the FSK modulator of FIG. 4A.

FIG. 4D illustrates the tones in the optical signal received at a thirdfilter stage in the FSK modulator of FIG. 4A.

FIG. 4E illustrates the tone in the optical signal exiting the FSKmodulator of FIG. 4A.

FIG. 5A illustrates a Mach-Zehnder interferometer filter that is usedfor each filter stage of FSK modulator of FIG. 4A according to oneembodiment of the invention.

FIG. 5B is a graphical representation of the optical power transferfunction of the Mach-Zehnder interferometer filter of FIG. 5A.

FIG. 5C is a graphical representation of the optical power function ofFIG. 5B after changing the optical path length of one of the arms of theMach-Zehnder interferometer filter of FIG. 5A.

FIG. 6 illustrates another embodiment of an FSK modulator according tothe invention.

FIG. 7 illustrates a Mach-Zehnder interferometer stage for an FSKmodulator according to an embodiment of the invention.

FIG. 8A illustrates a microring resonator filter that can be used as afilter stage of FSK modulator according to an embodiment of theinvention.

FIG. 8B is a graphical representation of the optical power transferfunction of the microring resonator filter of FIG. 8A.

FIG. 8C is a graphical representation of the optical power transferfunction of FIG. 8B after changing the optical path length of themicroring resonator of FIG. 8A.

FIG. 9 illustrates another embodiment of an FSK modulator according tothe invention.

FIG. 10 illustrates a microring resonator filter stage for an FSKmodulator according to an embodiment of the invention.

DETAILED DESCRIPTION

In brief overview, the invention relates to an FSK modulator and amethod for large-alphabet (i.e., many different transmissionfrequencies) FSK modulation at high symbol rates (e.g., greater than 40GHz). The method is based on filtering of a multi-tone optical sourcesuch as a mode-locked laser which provides a comb distribution of tones.A frequency-selective component selects for transmission a subset of thetones (e.g., “alternate” tones) in the comb distribution. A secondfrequency-selective component selects a subset of the tones from thecomb distribution provided by the first frequency-selective component.Still more frequency-selective components can be used according to thenumber of tones supplied by the multi-tone optical source to the FSKmodulator. The optical signal exiting the last frequency-selectivecomponent of the FSK modulator includes only a single tone whichcorresponds to the symbol to be transmitted. In one embodiment thefrequency-selective components are Mach-Zehnder interferometer filterstages as described in detail below. In another embodiment thefrequency-selective optical components are microring resonator filterstages as described in more detail below.

FIG. 4A illustrates an embodiment of an FSK modulator 38 according tothe invention. The modulator 38 includes multiple filter stages 42A,42B, 42C (generally 42) in serial communication. FIG. 4B illustrates theoptical spectrum (having a comb distribution) of a multi-tone opticalsignal propagating from a transmitter source (not shown) to the firstfilter stage 42A. Similarly, FIG. 4C illustrates the tones in theoptical signal received at the second filter stage 42B, FIG. 4Dillustrates the tones in the optical signal received at the third filterstage 42C and FIG. 4E illustrates the tone in the optical signal exitingthe FSK modulator 38.

During operation, the multi-tone optical signal having the discreteoptical frequency components, or tones, at frequencies f₁ to f₈,(generally f) is received from a transmitter source such as amode-locked laser. Each frequency component f is at a single, uniquefrequency which is separated from adjacent frequency components by afree spectral range (FSR). The phrase “single frequency” as used hereinincludes a narrow range of frequencies within the linewidth of a laserline or tone as is understood by those of skill in the art. Althoughonly eight frequency components are shown in the illustrated embodiment,the invention contemplates other embodiments in which other numbers oftones can be provided by the transmitter source and that other numbersof filter stages 42 can be included in the FSK modulator 38.

The first filter stage 42A selects, that is, “passes” tones of alternatefrequencies in the received multi-tone optical signal. Thus tones ateither a first alternating set of frequencies (i.e., odd-indexedfrequencies f₁, f₃, f₅ and f₇) or a second set of alternatingfrequencies (i.e., even-indexed frequencies f₂, f₄, f₆ and f₈) areselected to remain in the transmitted optical signal according to thevalue of a control signal applied to the stage 42A. For example, thecontrol signal may be a binary voltage signal having a first value and asecond value to select the first and second alternating sets offrequencies, respectively. As illustrated, the control signal has thesecond value, causing the first filter stage 42A to select theeven-indexed frequencies f₂, f₄, f₆ and f₈. The second filter stage 42Bselects tones of alternate frequencies provided by the first filterstage 42A. Thus either tones at frequencies f₂ and f₆ are selected ortones at frequencies f₄ and f₈ are selected according to an applied,control signal. As shown, the control signal has a value that causesfrequencies f₂ and f₆ to be selected. Finally, the third filter stage42C selects one of the two tones f₂ and f₆ provided by the second filterstage 42B according to an applied control signal. As illustrated, thecontrol signal has a value that causes a single tone at frequency f₆ tobe included in the FSK optical signal exiting the FSK modulator 38.

Mach-Zehnder Interferometer Filter Stages

FIG. 5A illustrates a Mach-Zehnder interferometer (MZI) filter 46 thatis used for each filter stage 42 according to one embodiment of the FSKmodulator 38. In one embodiment the MZI filters 46 are fiber-coupledlithium niobate electro-optic modulators. The MZI filter 46 has adifferential propagation time (i.e., differential delay) T definedbetween the two interferometer arms 50A, 50B (generally 50) according tothe difference in their optical path lengths. The corresponding opticalpower transfer function for the MZI filter 46 for a particular opticalfrequency f is given byP _(out) ∝P _(in)[1+cos(2πfT)]where P_(in) and P_(out) are the input and output optical powers,respectively.

In the frequency domain, the optical power transfer function of theinterferometer is cosinusoidal with frequency 1/T, as shown in FIG. 5B.By changing the optical path length of one of the arms 50 by half theperiod corresponding to the optical frequency (e.g., by using anelectro-optic phase modulator 52), the optical power transfer functionnear the frequency of the optical carrier is shifted so that the peaksof the shifted transfer function shown in FIG. 5C align approximatelywith the nulls of the unshifted transfer function shown in FIG. 5B. Forexample, by modulating a voltage applied to the electro-optic modulator52, the optical path length of the lower arm 50B is changed. Usingconventional high-speed optical modulation techniques, the optical powertransfer function of the MZI filter 46 can be shifted between the twooperating points shown in FIGS. 5B and 5C at a rate than exceeds 40 GHz.

FIG. 6 illustrates an embodiment of an FSK modulator 54 according to theinvention. A source laser (not shown) provides a number M of uniquetones separated by a frequency difference F. By way of example, thesource laser can be multiple continuous-wave lasers or a mode-lockedlaser having a repetition rate equal to the desired separation betweenthe FSK tones. (In an alternative embodiment, broadband optical noisesuch as that realized through amplified spontaneous emission from anoptical amplifier is transmitted through a Fabry-Perot etalon togenerate the multi-tone optical signal.) The multi-tone optical signalfrom the source laser is modulated using a number k of MZI filter stages58A to 58K (generally 58) where k=log₂ M. Filter stage 1 58A has adifferential delay of 1/(2F) seconds and a period of 2F. Thus filterstage 1 58A can be used to eliminate one-half of the M tones of thesource laser, i.e., the odd-indexed frequencies or the even-indexedfrequencies as described above. The particular tones that are filtereddepend on the differential phase shift imposed by an electro-opticmodulator 62A according to an applied control voltage V₁. Thus M/2 tonesare provided at the output of filter stage 1 58A. Filter stage 2 58B hasa differential delay of 1/(4F) so that the width of a transmission peakin the optical power transfer function in the frequency domain (see,e.g., FIG. 5B) is twice the width of a transmission peak in the opticalpower transfer function for the first filter stage 58A. Thus filterstage 2 58B can be used to select one-half of the tones provided byfilter stage 1 58A. The differential phase shift applied for filterstage 2 58B is responsive to the voltage V₂ applied to the electro-opticmodulator 62B. The voltage V₂ has one of four values corresponding todifferential phase shifts of 0, π/2, π and 3π/2. These values enable theoptical power transfer function to be shifted to one of four possiblepositions in frequency. Each subsequent filter stage 58 has adifferential delay equal to one-half of the preceding filter stage 58and twice the number of possible values for its control voltage as usedfor the preceding filter stage 58. In this manner, a single tone fromthe laser source remains in the optical signal exiting the k^(th) filterstage.

Preferably a group filter is used to reject all tones outside theoperating frequency range of the FSK modulator 54. In a preferredembodiment, the group filter is disposed between the source laser andthe FSK modulator 54. Alternatively, the group filter can be providedafter the FSK modulator 54. In either embodiment, all tones outside afrequency band defined by the lowest and highest frequencies of the Mtones are rejected. In other embodiments of the FSK modulator, thefilter stages 58 are arranged sequentially such that the differentialdelays of subsequent filter stages 58 are not monotonically decreasingwhile still maintaining the ability to select a single tone fortransmission.

Since each of the MZI filter stages 58 in the FSK modulator 54 can betuned at a rate that exceed 40 GHZ using existing electro-opticmodulation techniques, the tone available at the output of filter k canalso be selected at a rate exceeding 40 GHz. Only k filter stages arerequired to select one of M=2^(k) tones. Thus the modulation techniqueof the invention enables high-speed (greater than 40 GSymbol/s), largealphabet FSK modulation. The modulation technique can enable futurehigh-bandwidth (greater than 100 Gbit/s) free-space optical links wherelarge transmission alphabets are desirable to obtain high powerefficiency at receivers.

Environmental factors such as temperature fluctuations can cause therelative phase difference between the two interferometer arms 50A and50B of each filter stage 58 to drift or otherwise become unstable.Different techniques can be used to stabilize the filter stages 58during operation of the FSK modulator. According to one technique, anout-of-band optical signal is employed. For example, the out-of-bandoptical signal can be from an optical source operating at a differentwavelength (e.g., 1330 nm) than the wavelength of the FSK modulatedoptical signal (e.g., 1550 nm). The out-of-band optical signal ismonitored at each filter stage 58 at a low bandwidth relative to thesymbol rate but at a high bandwidth relative to the time scale ofenvironmental changes. Responsive “compensation” signals are generatedto slowly change the bias voltages of the electro-optic modulators 62,thereby counteracting any drift in the relative phase differences.According to another stabilization technique, each filter stage 58modulates the received (in-band) optical signal about the peak of itsoptical transfer function at a low modulation frequency (e.g., 1 KHz).The resulting doubled frequency (e.g., 2 KHz) at the output of eachfilter stage 58 is monitored to determine changes in the relative phasedifference between the two interferometer arms 50A and 50B and togenerate a compensation signal to modify the bias voltage of theelectro-optic modulator 62.

The high-speed FSK modulator 54 described above can be challenging toimplement due to the multi-level modulation required for the MZI filtersstages 58. In particular, filter stage 1 58A requires a differentialphase shift of 0 or π. Filter stage 2 58B requires a differential phaseshift of 0, π/2, π or 3π/2. The k^(th) filter stage 58K requires one of2^(k) phase shifts of the form nπ/2^(k-1), where n=0, 1, . . . ,2^(k)−1. The multi-level modulation can be achieved by driving theelectro-optic modulators 62 with an N-bit digital-to-analog converter(DAC) that provides voltages in the range of 0 to 2V_(π), where V_(π) isthe drive voltage required to obtain a differential phase shift of π inthe optical power transfer function. The speed at which modulation isachieved is limited by the speed of the DAC used to generate the drivevoltages for the electro-optic modulators 62.

An alternative method for FSK modulation according to the inventionachieves higher modulation rates. The phase shift imparted by anelectro-optic modulator is generally proportional to the product of anapplied voltage and an interaction length between the applied voltageand the optical signal propagating in a waveguide. Typically, theinteraction length corresponds to the length of an electro-opticmodulator electrode disposed alongside the waveguide. A k^(th) MZI stage66 can be fabricated as shown in FIG. 7 with k electrodes 70A to 70D(generally 70) of varying length instead of a single electrode. In theillustrated embodiment, the number k of electrodes 70 is four. As shown,each successive electrode 70 is one-half the length of the precedingelectrode 70. If each electrode 62 is driven with a binary voltagesignal (V_(b1), V_(b2), V_(b3) or V_(b4)) having a value of 0 or Vπ(where Vπ is the drive voltage required to obtain a π phase shift fromthe first electrode 70A), any of the desired FSK modulation levels forthe k^(th) MZI stage 66 can be achieved. All the electrodes 70 in theFSK modulator that have a common length receive the same binary voltageV_(b). Thus, for an FSK modulator having a number N of MZI stages, thebinary voltage V_(b1) is applied to the electrode of length L in eachMZI stage. Similarly, the binary voltage V_(b2) is applied to theelectrodes of length L/2 which are present in all but the first MZIstage. In a more generalized sense, the binary voltage V_(bx) is appliedto all electrodes of a corresponding length which are present in N−x+1of the MZI stages.

The binary voltages V_(bx) applied to the electrodes 70 are the samevoltage signals that would be used as digital inputs to the DACdescribed above. Thus the illustrated multi-electrode MZI filterarchitecture eliminates the need for the DAC and enables modulation atgreater speeds.

In other embodiments, the arrangement and operation of the electrodes 70differ from that shown in FIG. 7 and described above. For example, theelectrodes 70 need not be arranged serially according to amonotonically-decreasing length. Also, the electrodes 70 can be disposedon both arms 68A and 68B where the binary voltages can be different thanthose described above yet still achieve the number and value of themodulation levels for each MZI stage as described above. In still otherembodiments, the multi-electrode interferometer configurations can bedetermined according to an alternative translation from bits to tones.In such embodiments, a logical calculation of hits to drive level isperformed in addition to the digital to analog conversion.

Microring Resonator Filter Stages

According to an alternative to the embodiment of the FSK modulator 38shown in FIG. 4A, a microring resonator (MR) filter 74 shown in FIG. 8Ais used for each filter stage 42. The MR filter 74 includes a first(input) waveguide 78, a microring resonator 82 and a second (output)waveguide 86. The microring resonator 82 has a closed, loop form such asthe illustrated circular shape; however, the resonator 82 can also be inthe form of an ellipse, racetrack or other closed-loop shape that cansupport traveling wave resonant modes without high loss as is known inthe art. Optical power is coupled from the first waveguide 78 to themicroring resonator 82 by evanescent coupling. The resonator 82 supportsresonant wavelengths λ₀ given byλ₀=(2πRn _(e))/Nwhere R is the radius of the microring 82, n_(e) is the effectiverefractive index of the mode supported in the microring 82 and N is aninteger value, and which are not significantly attenuated due towaveguide material attenuation and other loss mechanisms. Energy in themicroring resonator 82 at the resonant frequencies is coupled to thesecond waveguide 86. Tones in the optical signal propagating in thefirst waveguide 78 that are not coupled into the microring resonator 82continue to propagate in the first waveguide 78.

The optical power transfer function of the MR filter 74 is shown in FIG.8B. By changing the optical path length of the microring resonator 82 byhalf the period, the optical power transfer function near the opticalcarrier is shifted by approximately half the FSR as shown in FIG. 8C.For example, the optical path length of the microring 82 is changed bychanging a voltage V applied to an electro-optic modulator electrode 90,allowing the modulation of the optical power transfer function at ratesthat can exceed 40 GHz.

MR filter stages 74 used in embodiments of the FSK modulator can haveradii of tens of microns or less. Thus the MR filter stages 74 are muchsmaller than the equivalent MZI filter stages described above.Potentially, a single MR filter stage 74 can be fabricated to a smalldimension to simplify the FSK modulator by achieving an FSR that is atleast as great as the modulator wavelength range. For example, for aC-band FSK modulator the operating wavelength range is approximately 30nm. Consequently, a single MR filter stage 74 having a microring 82 witha radius R of 5 μm or less is capable of selecting a single tone;however, fabrication of the desired number of electrodes 90 ofdecreasing size presents a significant challenge. Moreover, use of asingle MR filter stage 74 for a large number of tones requires both alarge FSR and a small passband. Consequently, the MR filter stage 74 isrequired to have a high quality factor (Q) than can limit how rapidlythe filter stage can be tuned, making an FSK modulator utilizingmultiple MR filter stages more desirable.

FIG. 9 illustrates another embodiment of an FSK modulator 94 accordingto the invention. A multi-tone optical signal having a number M ofunique tones is modulated using a number k of MR filter stages 98A to98K (generally 98) where k=log₂ M. Each filter stage 98 has a microring82 of a different radius (and different optical path length) so that theoptical power transfer functions have different FSRs. Filter stage 1 98Ais used to eliminate one-half of the M tones provided to the modulator94 according to the applied voltage V₁, filter stage 2 98B eliminatesone-half of the M/2 tones provided by filter stage 1 98A according tothe applied voltage V₂, and so on until after the k^(th) filter stage98K only a single tone from the multi-tone optical signal remains.

Due to the high finesse of the optical power transfer function of the MRfilter stages 98, the invention contemplates embodiments in which thenumber of filter stages 98 is less than k. For example, designconstraints such as modulation speed and alphabet size may make itdesirable to employ a reduced number of filter stages 98. In otherembodiments, the configuration of microrings 82 can be different fromthat shown in FIG. 9. For example, the microrings 82 can be positionednear each for direct evanescent coupling between adjacent microrings 82.Thus intervening waveguides (e.g., waveguides 78B, 78C, 86A and 86B) canbe eliminated.

FIG. 10 illustrates an MR filter stage 98C having three electro-opticmodulator electrodes 102A, 102B and 102C (generally 102). The arclengths of the electrodes 102 and the corresponding drive voltagesV_(b1), V_(b2), and V_(b3) are similar to the lengths of the linearelectrodes 70 and drive voltages of FIG. 7. Thus an FSK modulator can befabricated using multiple MR filter stages 98 with varying numbers andlengths of electrodes 102 to avoid the need for a DAC and therebyachieve greater modulation speeds.

While the invention has been shown and described with reference tospecific embodiments, it should be understood by those skilled in theart that various changes in form and detail may be made therein withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A frequency-shift keying modulator for opticalcommunications comprising: a first filter stage having an input opticalpath to receive a multi-tone optical signal having a plurality of tones,an output optical path and a first microring resonator having a firstoptical path length and being disposed between the input and outputoptical paths, the first microring resonator having an electro-opticmodulator disposed therein to modulate the first optical path length inresponse to a first control signal having a time-dependent signal valuedetermined according to a communications signal to be transmitted,wherein a subset of the tones in the multi-tone optical signal areprovided in a first filtered optical signal in the output optical pathof the first filter stage; and a second filter stage having an inputoptical path in optical communication with the output optical path ofthe first filter stage, an output optical path and a second microringresonator having a second optical path length and disposed between theinput and output optical paths, the second microring resonator having anelectro-optic modulator disposed therein to modulate the second opticalpath length in response to a second control signal having atime-dependent signal value determined according to the communicationssignal to be transmitted, the second control signal having a number oftime-dependent signal values that is twice a number of time-dependentsignal values of the first control signal, wherein a subset of the tonesin the first filtered optical signal are provided in a second filteredoptical signal in the output optical path of the second filter stage. 2.The frequency-shift keying modulator of claim 1 further comprising amulti-tone optical source in optical communication with the inputoptical path of the first filter stage.
 3. The frequency-shift keyingmodulator of claim 1 further comprising a group filter in opticalcommunication with one of the first and second filter stages.
 4. Afrequency-shift keying modulator for optical communications comprising:a first filter stage having an input optical path to receive amulti-tone optical signal having a plurality of tones, an output opticalpath and a first microring resonator having a first optical path lengthand disposed between the input and output optical paths, the firstmicroring resonator having at least one electro-optic modulatorelectrode disposed therein to modulate the first optical path length inresponse to a first control signal having a time-dependent signal valuedetermined according to a communications signal to be transmitted,wherein a subset of the tones in the multi-tone optical signal areprovided in a first filtered optical signal in the output optical pathof the first filter stage; and a second filter stage having an inputoptical path in optical communication with the output optical path ofthe first filter stage, an output optical path and a second microringresonator having a second optical path length and disposed between theinput and output optical paths of the second filter stage, the secondmicroring resonator having at least one electro-optic modulatorelectrode disposed therein to modulate the second optical path length inresponse to a second control signal having a time-dependent signal valuedetermined according to a communications signal to be transmitted,wherein the numbers of electro-optic modulator electrodes in the firstand second microring resonators are different, a length of at least oneof the electro-optic modulator electrodes of one of the microrings beingless than the lengths of the electro-optic modulator electrodes of theother microring, wherein at least one tone of the multi-tone opticalsignal is selected for transmission through the first and second filterstages according to the first and second control signals.
 5. Thefrequency-shift keying modulator of claim 4 further comprising amulti-tone optical source in optical communication with the first filterstage.
 6. The frequency-shift keying modulator of claim 4 wherein adifference in an applied voltage at a maximum value of thetime-dependent signal values of the first and second control signals isbased on a voltage which, when applied to a longest electro-opticmodulator electrode of one of the microrings, causes a differentialphase shift of one-half the free spectral range of an optical powertransfer function of the respective microring.